U.S. Pat. No. 5,023,458 to Benveniste et al. concerns an ion beam control system. The '458 patent discloses an ion implantation source that emits ions from multiple apertures to produce multiple beam portions which combine to form a large diameter beam. One goal of the '458 patent is to generate an ion beam approximately the size of a workpiece from multiple beam portions which can be turned on and off in a controlled fashion to control beam intensity. This was accomplished by controlling beam neutralization of the ion beam as a function of position within the cross section of the beam. The disclosure of the '458 patent to Benveniste et al. is incorporated herein by reference.
In an ion beam implanter of the type disclosed in the '458 Benveniste et al. patent, neutralization of the beam is achieved by interaction between ions within the beam and a residual gas along the beam transport line. Beam ions ionize the residual gas and liberate electrons having a low energy that are captured within the potential well of the beam. Slow residual gas ions are expelled from the beam. The space charge of the beam is neutralized by the low-energy electrons and this enables the beam to be transported from a source to a workpiece without significant loss in beam definition. Since ion beam transport is dependent upon the presence of low-energy neutralizing electrons, it is important to assure electrons are not unintentionally extracted from the ion beam during beam transport.
Prior art ion beam sources have an extraction electrode for attracting positively charged ions from an ion source. A suppression electrode biased at a slight negative potential prevents passage of neutralizing electrons from the ion beam back to the highly positive extraction electrode. But for the presence of the suppression electrode, the neutralizing electrons would be drawn back to the extraction electrode, thus deneutralizing the beam and causing beam blow-up.